The present invention generally relates to optical fiber couplers used in optical communication, measurement, etc. and more particularly, to a method of producing an optical fiber coupler, in which glass portions of a plurality of optical fibers are fusion bonded to each other and stretched.
When an optical communication system or an optical data link network is constructed, an optical directional coupler for distributing, at a desired ratio, an optical signal delivered from a light source is a vital constituent element. One example of the optical directional coupler is an optical fiber coupler. Usually, the optical fiber coupler is produced as follows. Namely, a plurality of optical fibers are stranded or arranged in parallel and then, glass portions of the optical fibers are heated by using a heat source such as an acetylene gas burner so as to be fusion bonded to each other integrally. Subsequently, the integral glass portions are further heated and stretched under a constant tension so as to be tapered.
In a known method disclosed in, for example, Japanese Patent Laid-Open Publication No. 64-21405, when glass portions of a plurality of optical fibers, which are secured by a pair of clampers, are heated by a gas burner acting as a heat source so as to be fusion bonded to each other and stretched, a gas burner 6 having a single gas nozzle 7 as shown in FIG. 3 is reciprocated in parallel with an axial direction of the optical fibers. This is because in order to obtain an optical fiber coupler having excellent characteristics, it is desirable that the glass portions of the optical fibers are uniformly heated over a length of about 5 mm so as to be fusion bonded to each other and stretched. However, the known method has the following drawbacks (1) and (2).
(1) Since reciprocation of the gas burner 6 is stopped at one end of the glass portions and then, is started towards the other end of the glass portions, heating period of the opposite ends of the glass portions becomes longer relatively and thus, heating temperature of the opposite ends of the glass portions is likely to become higher than that of the remaining part of the glass portions. Therefore, in the fusion bonding process, joining degree of the glass portions of the optical fibers rises only at the opposite ends of the glass portions. Meanwhile, in the stretching process, only the opposite ends of the glass portions are stretched sufficiently. Thus, the stretched glass portions have an uneven shape in the axial direction of the optical fibers. As a result, an optical fiber coupler having excellent characteristics cannot be produced at high reproducibility.
(2) In the stretching process, the glass portions are heated under a constant tension so as to be stretched. However, if such a phenomenon takes place during the stretching process that the tension varies or heating temperature varies due to wavering of a flame, etc., stretching speed changes only at a location of the glass portions, which is being heated by the gas burner 6 at the time of occurrence of the variation, relative to the remaining locations of the glass portions. Therefore, the stretched glass portions are likely to have an uneven shape in the axial direction of the optical fibers. As a result, an optical fiber coupler having excellent characteristics cannot be produced at high reproducibility.